A breathing apparatus of this type is disclosed in United States patent application publication 2008/0127979 A1. Ambient air is drawn in via a filter upstream of the blower and conveyed to a hood or a breathing mask via a hose. The filter serves to filter out pollutants present in the ambient air. Multiple filters are normally operated in parallel to provide sufficient fresh breathing air. Because the filter resistance changes during the course of use, the blower is operated with previously measured characteristic lines with which the filter resistance can be estimated and a predetermined output for the volume flow can be set. As a result of the breathing air flow, a certain excess pressure develops in the hood or the breathing mask, which prevents the infiltration of harmful gases into the breathing air, and the breathing air flow must be set so that sufficient carbon dioxide can be flushed out. Typically, a breathing air flow of approximately 135 l/min is used.
The characteristic lines stored in the known breathing apparatus correspond to the circumstances during calibration and the ambient conditions during calibration. The temperature and the ambient pressure are important parameters. Because the known breathing apparatus can be used at different elevations, the density of the air drawn in changes and thus the blower output changes.
In a breathing apparatus known from United States patent application publication 2009/0266361 A1, it is suggested to determine the ambient pressure with a pressure sensor, to supply the measured value to the motor control and to incorporate it into the control of the motor. This requires an additional pressure sensor which needs to be monitored and maintained.